On Sat, Mar 24, 2007 at 09:37:07PM +0000, Gregory Stark wrote: > It sounds like a fine idea from the point of view of flexibility. But as far > as faster... I guess it depends on how often HeapTupleSatisfiesVisibility is > used in contexts where the compiler is able to optimize away the conditionals > or the cpu is able to predict them accurately. I suspect in the cases where we > actually care--scans where it's being called thousands of times quickly--the > latter is quite effective.
Dunno, this test by someone suggests that calling through a function pointer doesn't cost anything at all, if anything it's faster. Though that may differ on other machines obviously. http://gcc.gnu.org/ml/gcc/2004-06/msg01308.html In any case, looking at the code I don't think the compiler can optimise HeapTupleSatisfiesVisibility at all ever, because it's never specified explicitly. > Function pointers are notoriously hard to optimize around and can actually > make the surrounding code harder to optimize as well especially since we > compile with -fno-strict-aliasing. So whether it's faster or slower may depend > a lot on the specific call site. The costs of -fno-strict-aliasing are probably measurable, but I don't think we can do anything about that... Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate.
signature.asc
Description: Digital signature
